const keyword scope in Javascript

Question

1. >>> const a = 2
2. >>> var a = 3
3. >>> a = 4
4. >>> a // print 2

Why the operation line 3 is allowed? const

Answer 1

This is is just how const works (or doesn't work):

Creates a constant¹ that can be global or local to the function in which it is declared. Constants follow the same scope rules as variables [.. and cannot share a name] with a function or a variable in the same scope.

Firefox [..] throws a TypeError if you redeclare² [which is different than re-assigning] a constant. None of the major browsers produce any notices or errors^2,3 if you assign another value to a constant [..] but the reassignment is unsuccessful (only) in Firefox and Chrome (at least since version 20).

Note that const is not part of the ECMAScript 5 specification and the JavaScript 1.5 semantics will be re-defined in ECMAScript 6.

Behavior will vary across browser implementations with respect to support and re-declaration/re-assignments semantics.

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¹ In IE 9, using const a = 2 results in

"Syntax error"

² In FF 14, const a = 2; var a = 3; a = 4; a, when evaluated as a single program, results in

TypeError: redeclaration of const a

which is different than executing each line one-at-a-time in the REPL. I suspect this is because var is hoisted above the const and because a const "cannot share a name with a function or variable in the same scope".

³ In Chrome 21, const a = 2; var a = 3; a = 4; a evaluates to 2 with no warning or message.

Answer 2

const scope is defined as 'block scoped' (the scope of which, is restricted to the block in which it is declared).

MDN documentation:

Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.

Regarding your specific issue: First as comments said const is relevant in ES6. I don't know about you but i get (typing your line 2: var a = 3;): SyntaxError: Identifier 'a' has already been declared so your example is not quite possible.